Signal identification in optical communications networks

ABSTRACT

Signals in an optical communications network, such as optical channels in an optical WDM network for example, are each identified by at least two low frequency dither tones with which the signal is modulated. The dither tones alternate with a predetermined periodicity to produce a cyclically repeated sequence of dither tones. A network parameter, such as a channel identifier for example, is obtained by the detection of the particular combination of dither tones in the sequence. To detect a number of network parameters a signal is modulated with a number of cyclically repeated sequences of dither tones each uniquely identifying a respective network parameter. In some implementations each dither tone in a cyclically repeated sequence of dither tones is repeated with substantially the same phase and coherent averaging is performed over a number of periods to detect dither tones on low power signals.

RELATED APPLICATION

This application is a Continuation-In-Part of U.S. patent applicationSer. No. 10/263,959 to Wan et al, entitled “Channel Identification inCommunications Networks” filed Oct. 04, 2002, and claims benefit fromU.S. patent application Ser. No. 10/067,748 filed on 8 Feb. 2002; U.S.patent application Ser. No. 09/972,991 to Wan et al, filed on 10 Oct.2001; and from Canadian Application Serial No. 2,358,382 to Wan et al,filed on 5 Oct. 2001.

FIELD OF THE INVENTION

This invention relates to channel identification in communicationsnetworks or systems. The invention is generally applicable tocommunications networks or systems using any type of communicationsmedia, such as optical, wireless, or wired communications systems, butis particularly applicable to, and is described below in the context of,optical WDM (wavelength division multiplex) communications networks orsystems.

BACKGROUND

In optical WDM communications networks or systems it has been proposedto identify each of a plurality of optical signals or channels, each atan individual optical wavelength, with a respective relatively lowfrequency dither tone with which the intensity or amplitude of therespective optical signal is modulated. For example, in a WDM network inwhich optical signals are each modulated with data at a high bit rate,for example 2.5 Gb/s or more, each optical signal may also be modulatedwith a respective dither tone in a relatively low frequency range, forexample about 10 kHz to about 100 kHz or more. The dither tonemodulation can be provided with a specific modulation depth, thereby notonly providing channel identification but also power level informationfor the optical signal, thereby to facilitate functions such asperformance monitoring and fault management in the network.

Examples of such proposals are disclosed in Hill et al., “A TransportNetwork Layer Based On Optical Network Elements”, IEEE Journal ofLightwave Technology, Volume 11, No. 5/6, pages 667-679, May/June 1993,and in Roberts U.S. Pat. No. 5,513,029, issued Apr. 30, 1996 andentitled “Method And Apparatus For Monitoring Performance Of OpticalTransmission Systems”.

Heismann et al., “Signal Tracking And Performance Monitoring InMulti-Wavelength Optical Networks”, 22nd European Conference on OpticalCommunication—ECOC'96, pages 3.47 to 3.50, 1996 also discloses such aproposal in which a pilot tone which acts like a dither tone can furtherbe modulated using frequency-shift keying (FSK) with an additionaldigital signal providing digital information, such as optical routinginformation. For example, this article discloses FSK modulation of tonesat 10 kHz and 12 kHz each with ±500 Hz frequency excursions inaccordance with respective 100 b/s digital signals.

While such proposals provide the advantage that optical channels can beidentified and their power levels monitored without detection anddemodulation of the optical channel itself and without serious adverseeffects on the optical channels, they fail to meet increasingrequirements of WDM networks in several respects.

More particularly, such proposals provide a limited number ofdistinguishable dither tones, each of which identifies a respectiveoptical channel so that the WDM network can have only a similarlylimited number of optical channels. In addition, detection of such tonescan be very difficult. For example, an optical fiber in a WDM networkmay carry up to 32 optical channels within a wide optical dynamic rangeof for example 30 dB (a dynamic range of 60 dB for the correspondingelectrical signals) or more. Detection of a dither tone for an opticalchannel at the lower end of this dynamic range is very difficult in thepresence of possibly many other optical channels higher in this dynamicrange, because the latter constitute noise for the detection process.

Further, dither tone detection can be complicated by the presence ofother interference, such as interference tones that occur with afrequency spacing of about 8 kHz in the case of SONET (synchronousoptical network) communications. Furthermore, many optical networkscarry non-SONET (non-Synchronous Optical Network) signals, such asGigabit Ethernet and proprietary signals that have interference toneswith unknown or changing frequencies. Such tones can cause interferenceand this also complicates dither tone detection.

A need therefore exists for an improved method of and apparatus forchannel identification which can facilitate robust detection of dithertones for identification of larger numbers of channels in acommunications network, in particular an optical WDM network.

SUMMARY OF THE INVENTION

According to one aspect of this invention there is provided a signalidentification method in an optical communications network. The methodinvolves modulating an optical signal with a cyclically repeatedsequence of low frequency dither tones having a predeterminedperiodicity. The method involves performing a sequence of frequencyanalysis operations on the optical signal to produce respectivefrequency analysis results. The method also involves detecting the lowfrequency dither tones using the respective frequency analysis resultsproduced by the frequency analysis operations.

In some embodiments of the invention a network parameter is identifiedusing the detected low frequency dither tones.

In some embodiments of the invention the cyclically repeated sequence oflow frequency dither tones includes at least one set of low frequencydither tones modulated simultaneously. In such embodiments, the methodinvolves identifying a network parameter by detecting at least one ofthe low frequency dither tones from each set. This provides a robustmechanism for detecting network parameters when some of the dither tonescannot be detected.

In some embodiments of the invention the optical signal is modulatedwith at least one other cyclically repeated sequence of low frequencydither tones having a respective predetermined periodicity. In suchembodiments the method involves: for each cyclically repeated sequenceof low frequency dither tones, detecting the low frequency dither tonesusing the respective frequency analysis results produced by thefrequency analysis operations, and identifying a respective networkparameter from the detected low frequency dither tones.

In some embodiments of the invention each cyclically repeated sequenceof low frequency dither tones uniquely identifies its respective networkparameter.

In some embodiments of the invention each network parameter is one of achannel ID (IDentifier), a link ID, an optical fiber ID, a fiber sectionID, an optical band ID, a source node ID, a destination node ID, and abundle ID.

In some embodiments of the invention the cyclically repeated sequencesof low frequency dither tones are modulated in one of the followingways: (a) simultaneously; (b) consecutively; and (c) using a combinationof (a) and (b).

In some embodiments of the invention each low frequency dither tone inthe cyclically repeated sequence of low frequency dither tones isrepeated with substantially the same phase at the predeterminedperiodicity. In such embodiments the method involves performing coherentaveraging of the frequency analysis results over a plurality offrequency analysis operations to detect the low frequency dither tones.

According to another aspect of this invention there is provided a methodof modulating an optical signal in an optical communications network.The method involves modulating the optical signal with a cyclicallyrepeated sequence of low frequency dither tones having a predeterminedperiodicity.

In some embodiments of the invention the optical signal is modulatedwith at least one other cyclically repeated sequence of low frequencydither tones having a predetermined periodicity. This allows a number ofthe above network parameters to be identified by the detection of thesequences.

In some embodiments of the invention at least one of the low frequencydither tones is modulated at least twice within the sequence.

In some embodiments of the invention the cyclically repeated sequence oflow frequency dither tones comprises a plurality of sets of at least onelow frequency dither tone. At least two of the plurality of sets have adifferent number of low frequency dither tones.

According to another aspect of this invention there is provided adetection arrangement for an optical communications network. Thearrangement has a detector for detecting an optical signal modulatedwith a cyclically repeated sequence of low frequency dither tones havinga predetermined periodicity. The detector also has a processor forperforming a sequence of frequency analysis operations on the opticalsignal to produce respective frequency analysis results and fordetecting the dither tones using the respective frequency analysisresults produced by the frequency analysis operation.

According to another aspect of this invention there is provided amodulating arrangement for modulating an optical signal. The arrangementhas means for producing a cyclically repeated sequence of low frequencydither tones having a predetermined periodicity. The arrangement alsohas a modulator for modulating the optical signal with the cyclicallyrepeated sequence of low frequency dither tones.

In some embodiments of the invention the arrangement also has means forproducing at least one other cyclically repeated sequence of lowfrequency dither tones having a predetermined periodicity. In suchembodiments the modulator also has means for modulating the opticalsignal with the other cyclically repeated sequences of low frequencydither tones.

In some embodiments of the invention the arrangement has at least oneprogrammable frequency source for generating the low frequency dithertones.

In some embodiments of the invention the arrangement has at least twoprogrammable frequency sources for generating the low frequency dithertones and at least one selector for alternately selecting the lowfrequency dither tones.

In some embodiments of the invention the arrangement has a DAC (Digitalto Analog Converter) for generating the low frequency dither tones.

According to another aspect of this invention there is provided anoptical system having a modulating arrangement for generating acyclically repeated sequence of low frequency dither tones having apredetermined periodicity and for modulating an optical signal with thecyclically repeated sequence of dither tones. The system also has adetection arrangement for: (a) detecting the signal; (b) performing asequence of frequency analysis operations on the optical signal toproduce respective frequency analysis results; and (c) detecting the lowfrequency dither tones using the respective frequency analysis resultsproduced by the frequency analysis operation.

In some embodiments of the invention the system has a global clock forsynchronizing time intervals for modulating the signal with the dithertones at the modulating arrangement and time intervals for detection ofthe low frequency dither signal at the detection arrangement.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further understood from the following descriptionby way of example with reference to the accompanying drawings, in which:

FIG. 1 illustrates an optical multiplexer arrangement with reference towhich a problem addressed by an embodiment of the invention isexplained;

FIG. 2 diagrammatically illustrates possible power spectral densities ofsignals of the arrangement of FIG. 1;

FIG. 3 illustrates successive bursts of dither tones providing a channelidentification;

FIG. 4 a illustrates one form of dither tone generator and modulatingarrangement in accordance with an embodiment of the invention;

FIG. 4 b illustrates another form of dither tone generator andmodulating arrangement in accordance with another embodiment of theinvention;

FIG. 4 c illustrates another form of dither tone generator andmodulating arrangement in accordance with another embodiment of theinvention;

FIG. 5 illustrates one form of detection arrangement in accordance withan embodiment of the invention;

FIG. 6 a illustrates synchronized alternating of dither tones in anetwork;

FIG. 6 b illustrates an FFT spectrum for synchronized switching ofdither tones in a network;

FIG. 6 c illustrates an FFT spectrum for unsynchronized switching ofdither tones in a network;

FIG. 7 illustrates the dither tone generator and modulating arrangementin accordance with a modification to the embodiments of the invention;

FIG. 8 illustrates a hardware windowing implementation;

FIG. 9A shows dither tones generated by DDS1 of FIG. 8; and

FIG. 9B shows dither tones with a hardware windowing technique beingapplied.

DETAILED DESCRIPTION

As indicated initially above, embodiments of the invention are describedbelow in the context of an optical WDM network, but the invention isalso generally applicable to, and the described embodiments of theinvention may be adapted for operation in, other types of communicationsnetwork.

As described above, one disadvantage of known channel identificationproposals is that each optical channel is identified by a respectivedither tone, and there is a limited number of dither tone frequenciesand hence a similarly limited number of optical channels in the WDMnetwork. While this number can be increased by increasing a frequencyrange over which the dither tones extend and/or by decreasing afrequency spacing of the dither tones within this range, such stepsinvolve other disadvantages, such as an increased potential forinterference with data signals and/or increased difficulty indistinguishing the dither tones from one another.

In embodiments of this invention, this disadvantage is greatly reducedor eliminated by using a combination of two or more low frequency dithertones for identification of each optical channel. For example, anoptical WDM network may provide 1600 dither tones in a frequency rangefrom about 48 kHz to about 64 kHz with a constant separation orfrequency spacing of 10 Hz between adjacent dither tones. A combinationof, for example, two such tones is used to identify each opticalchannel, so that the number of channels which can be identified isincreased, from 1600 using one such tone to identify each channel as inthe known proposals, to the order of 2.5 million. It is to be clearlyunderstood that other ranges of frequencies can be used for the lowfrequency dither tone. For example, in some implementations thefrequencies are in the kHz range and/or the MHz range.

A combination of a greater number of dither tones, for example three ormore, can alternatively be used to identify individually an even greaternumber of optical channels, so that the WDM network can have a virtuallyunlimited number of individually identifiable optical channels.Similarly, a combination of a greater number of dither tones can be usedin a redundant manner to increase reliability or robustness of thechannel identification, even in the case of low optical power levels andin the presence of interference. For example, each optical channel canbe identified by a respective combination of three dither tones, thechannel being detected by detection of at least any two of the threedither tones.

For simplicity in the following description it is assumed that eachoptical channel is identified by a respective combination of two dithertones. The nature of the combination of the dither tones is discussedfurther below.

Also, in order to increase the robustness of the channel identificationin the presence of interference, some constraints may be placed on theparticular selections of dither tones used to identify each opticalchannel. For example, with SONET interference tones occurring with afrequency spacing of about 8 kHz as described above, the dither tonesselected for each channel identification may be selected to avoidspacings of about 8 kHz between them, so that at worst only one of thetwo (or more) dither tones is subject to these interference tones.Furthermore, some optical networks carry non-SONET signals, such asGigabit Ethernet and proprietary signals that have interference toneswith unknown or changing frequencies. In such networks the multipletones are used for detecting a signal and overcome interference effects.As will be discussed in further details below, in such networks M dithertones are assigned to a channel, where M is an integer satisfying M≧2.In some embodiments of the invention, a channel is identified when oneor more of the M dither tones are detected.

The combination of the dither tones for each channel identification ispreferably an alternation of the two dither tones (or a cyclicrepetition for a sequence of more than two dither tones), each dithertone being modulated onto the respective optical channel in turn for apredetermined period as further described below. Thus the dither tonesare alternately (or cyclically) switched to modulate and therebyidentify the respective optical channel.

Although such switching of the dither tones is preferred as discussedfurther below, other ways of combining the dither tones are possible.For example, the dither tones for identifying each channel may be summedand the respective optical channel modulated with the resulting summedsignal. However, this is not preferred because this composite modulationundesirably produces greater closure of the “eye” for detection of thehigh speed data signal carried by the optical channel. Such eye closureis further increased using a sum of more than two dither tones for eachchannel identification.

It can be appreciated that, as in the FSK modulation of a single tonechannel identifier as described in the article by Heismann et al.referred to above, the two or more dither tones used for channelidentification as described here can be further extended foradditionally carrying low speed data in various ways.

As discussed above, a significant difficulty with known proposals forchannel identification using dither tones arises from a wide dynamicrange of optical signals which can occur in an optical WDM network. Thisis further described below by way of a very simple example representedby FIGS. 1 and 2.

Referring to FIG. 1, an optical multiplexer 10 is illustrated as beingsupplied with two optical signals on optical paths 12 and 14, andproducing a multiplexed optical signal on an optical path 16. Theoptical signal on the path 12 is assumed to comprise an optical channelhaving a wavelength λ1, this channel being identified by two alternatingdither tones f1 a and f1 b in the manner described above. The opticalsignal on the path 14 is assumed to comprise an optical channel having awavelength λ2, this channel being identified by two alternating dithertones f2 a and f2 b also in the manner described above.

FIG. 2 represents part of a graph of power spectral density (PSD) versusfrequency, for signals of the optical channels at the wavelengths λ1 andλ2; FIG. 2 in particular illustrates a small part of the frequency rangewhich includes the dither tones f1 a, f1 b, f2 a, and f2 b which areassumed for convenience of illustration to be close together. As dithertone detection is typically performed using an FFT (Fast FourierTransform) process which produces total energy or power results forrespective frequency bins or adjacent frequency ranges, the frequencyaxis in FIG. 2 is labelled to show such frequency bins numbered n−1 ton+7 where n is an integer. For example, FIG. 2 illustrates thesefrequency bins as being centered at frequencies 10 Hz apart,corresponding to a constant frequency spacing of adjacent dither tonesof 10 Hz as stated above by way of example, the FFT process or operationbeing performed over a period T which is the inverse of the frequencybin periodicity, so that in this case (1/T)=10 Hz.

As illustrated in FIG. 2, the dither tones f1 a and f1 b are at thecentres of the frequency bins n and n+3 respectively, and the dithertones f2 a and f2 b are at the centres of the frequency bins n+2 and n+6respectively.

The high speed data carried by the optical channels is typically NRZ(non-return to zero) data having a sinc ((sin x)/x) frequencycharacteristic, for which the PSDs of the signals for the opticalchannels having the wavelengths λ1 and λ2 are also illustrated in FIG.2. The optical signals can have relative optical powers which may beanywhere within a wide dynamic range, typically a range of about 30 dBor more, corresponding to (electrical) PSDs within a range of about 60dB or more as represented in FIG. 2 by a vertical dashed line. Withinthe respective frequency bins of the FFT process, the high speed datasignal components of the optical channels constitute noise whichdetracts from the dither tone detection.

By way of example, it is assumed that the optical channel at thewavelength λ1 has a relative optical power near the low end of thedynamic range (for example this optical channel may have traversed alarge number of attenuating optical components), and that the opticalchannel at the wavelength λ2 has a relative optical power near the highend of the dynamic range (for example it may have been supplied from alocal modulated laser source). Over the range of the frequency bins thecorresponding high speed data PSDs of these channels are substantiallyconstant and are represented by horizontal lines labelled λ1 and λ2respectively in FIG. 2.

In comparison, the PSDs of the dither tones f1 a and f1 b for therelatively weak optical channel at the wavelength λ1 are very small, asshown in FIG. 2, so that these dither tones can be very difficult todetect. It is observed that, as shown in FIG. 2, the PSDs of the dithertones f1 a and f1 b for the optical channel at the wavelength λ1, andlikewise the greater PSDs of the dither tones f2 a and f2 b for theoptical channel at the wavelength λ2, are not generally equal. This isbecause in the embodiments described above there is no synchronizationbetween the alternating periods for which the dither tones modulate therespective optical channels, so that each dither tone of each channelcan be present during an arbitrary part of the FFT period T.

It can be appreciated that the difficult problem of detecting the dithertones, e.g. f1 a and f1 b, of a weak optical channel is exacerbated inthe event, as may be typical, that the optical path 16 carries multipleoptical channels with high relative optical powers. For example, in anoptical WDM network each such optical path may carry up to 32 opticalchannels.

One way in which this problem can potentially be reduced is to decreasethe width of each frequency bin in the FFT process, thereby reducing thenoise component within each frequency bin due to the optical channels.This corresponds to an increase in the period T of the FFT process andthe number of frequency bins within a given frequency range, therebyconsiderably increasing computational and memory requirements for theFFT process, and also increasing a delay for detection of the dithertones. For the wide dynamic range indicated above, the period T may needto be of the order of 100 seconds, and these requirements and thecorresponding delay are increased to such an extent that this approachbecomes impractical.

It can be appreciated that this also necessitates a very precisegeneration of each dither tone. For example, a period T of 100 secondscorresponds to a frequency bin width of 0.01 Hz, requiring asubstantially better precision than this for generation of each dithertone.

Although these difficulties are very significant for the extremecondition of optical signals being at opposite ends of the wide dynamicrange as illustrated in FIG. 2, it can be realised that in mostsituations such extreme conditions will not apply. For optical signalsall of which are within a smaller dynamic range of for example about 20dB, it can be practical to detect the dither tones for all of theoptical channels using an FFT process with a period T of for example 1second and frequency bins of width 1 Hz, considerably reducing the FFTcomputational and memory requirements and the detection delay in suchmore usual conditions.

In view of these considerations, in an embodiment of the invention asdescribed below an FFT process is used with a period T, for example 1second, which is sufficiently short to be practical in terms ofcomputation, memory, and delay requirements and which in many cases ofoptical channels having typical optical power levels is sufficient topermit their dither tones to be detected within this FFT period, i.e. ina single FFT operation. This FFT process is supplemented by coherentaveraging of the FFT results over longer periods, i.e. over a plurality,possibly many, FFT operations, enabling dither tones of channels atlower relative powers also to be detected even where different ones ofthe optical channels have powers at both extremes of the dynamic range,extending over the maximum optical dynamic range of for example 30 dB.

To permit this coherent averaging, the dither tones are continuouslygenerated, and the alternating switching between the two dither tonesidentifying each optical channel has precisely controlled periods, sothat a dither tone detector can determine precisely a phase relationshipbetween successive bursts of each dither tone, as further describedbelow. In addition, the dither tones are generated with a desiredaccuracy, conveniently all being derived from a single, high frequency,stable oscillator.

By way of example, it is assumed that a duration t of a dither toneswitched alternately for modulation and hence identification of arespective optical channel is the same for all dither tones and for alloptical channels. Conveniently, this duration t may be of the order of 1second. FIG. 3 illustrates consequent successive bursts of the dithertones f1 a and f1 b which are used for modulation and identification ofthe optical channel having the wavelength λ1 as described above.

Referring to FIG. 3, the respective optical channel having thewavelength λ1 is modulated alternately as described above with thedither tones f1 a and f1 b. At a switching time t0, there is a switch ofthe modulating dither tone from f1 a to f1 b. Subsequently, at aswitching time t1 there is a switch of the modulating dither tone fromf1 b back to f1 a, at a switching time t2 there is a switch of themodulating dither tone from f1 a back to f1 b, and so on. Each dithertone burst has a duration t, i.e. the switching times t0, t1, t2, and soon occur periodically with the time spacing t.

Although there may be a phase discontinuity between the modulatingdither tones at the respective switching times, the facts that theseswitching times occur with the periodicity t and each dither tone isproduced continuously mean that there is a precisely determinable phaserelationship between successive bursts of each dither tone. Thus thereis a phase difference of 2πtf between the end of each burst of a dithertone and the start of the next burst of the same dither tone after aninterval t, where f is the frequency of the respective dither tone. Thusfor the dither tone f1 a this phase difference, between the switchingtimes t0 and t1, is 2πt(f1 a), and for the dither tone f1 b this phasedifference, between the switching times t1 and t2, is 2πt(f1 b).

Knowing the periodicity t, each dither tone detector can accordinglydetermine this phase difference for each dither tone, and use thedetermined phase difference for coherent averaging of the FFT resultsfor the respective dither tone over a plurality, possibly a largenumber, of FFT processing periods or FFT operations. The noise energydue to the optical signals over such periods is not similarly coherent,so that the coherent averaging, which is an accumulation of FFT resultsfor each respective frequency bin in accordance with amplitude and phaseover time, enhances the detection of the respective dither tone relativeto this noise.

More particularly, for detecting each dither tone, each FFT operationproduces a phase and amplitude result (for example represented by acomplex number) for the respective frequency bin. For coherent averagingover successive FFT operations, a current result or average can be phaseshifted in accordance with the phase difference between successivebursts of this dither tone as discussed above, and the result for thenext FFT operation for the same frequency bin added in accordance withits phase and amplitude (i.e. a vector addition). This accumulation canbe carried out in accordance with any desired averaging process, forexample using windowing or weighting of the FFT results. Over a desiredaveraging period, this coherent averaging distinguishes between a dithertone of a weak optical channel and noise.

Although an FFT is referred to above and is preferred because it enablesphase and amplitude results to be produced for all of the dither tonefrequencies for each FFT period T, it can be appreciated that otherforms of frequency analysis may be used to produce phase and amplituderesults for the respective dither tone frequencies, either individuallyfor different frequency analysis operation periods (for example, using aDiscrete Fourier Transform process) or collectively within a singleperiod.

FIG. 4 a illustrates one form of dither tone generator and modulatingarrangement, which can be used in an embodiment of the invention.Referring to FIG. 4 a, an optical channel is provided on an opticalfiber or path 20 from a modulated laser source 22, and is supplied viaan optical modulator 24 and an optical tap 26 to an ongoing optical path28. The source 22 provides the optical channel at a desired opticalwavelength and modulated with data to be carried by the optical channel,typically at a high bit rate of for example 2.5 Gb/s. The datamodulation can alternatively be carried out separately from the source22, for example on the optical path 20, or using the optical modulator24, or on the optical path 28 after the optical tap 26, the opticalmodulator 24 in the latter case modulating an optical carrier for theoptical channel. In any event, the optical modulator 24 providesintensity modulation of the optical channel for channel identificationas described below.

The optical tap 26 supplies a small portion, e.g. 5%, of the opticaloutput of the modulator 24 to an optical detector 30, whose electricaloutput is amplified by an AGC (automatic gain controlled) amplifier 32.An output of the amplifier 32 is supplied via a low pass filter (LPF) 34to an analog-to-digital converter (ADC) 36, and via a band pass or highpass filter (HPF) 38 and an amplifier 40 to an ADC 42. The ADCs 36 and42 produce digital signals, which are supplied to a digital signalprocessor or microprocessor (μP) 44.

An oscillator 46 provides a stable source of a signal, for example at afrequency of 50 MHz, which is supplied to the microprocessor 44 and toeach of a plurality of direct digital synthesizers (DDSs) or otherprogrammable frequency sources 48. Each DDS 48 is arranged to produce,under programmed control of the microprocessor 44, a respective one ofthe dither tones on a respective input to a selector 50. An output ofthe selector 50 is coupled via a controlled gain amplifier 52 and acapacitive coupling to a control input of the optical modulator 24. Theselector 50 and the gain of the amplifier 52 are controlled by themicroprocessor 44.

In operation, each DDS 48 is arranged to produce continuously arespective one of the dither tones to be used for identification of therespective optical channel; for example the dither tones f1 a and f1 bfor the optical channel λ1 as described above can be produced each by arespective one of two DDSs 48 at the source of this optical channel. Theselector 50 is controlled by the microprocessor 44 to alternately selectthese dither tones with the periodicity t as described above, wherebythese tones are modulated onto the optical channel by the opticalmodulator 24. In the case of more than two dither tones used for channelidentification, there is a correspondingly increased number of DDSs 48and selector inputs, and the selector 50 is controlled by themicroprocessor 44 to select the respective dither tones in a cyclicallyrepeated periodic sequence.

The LPF 34 and ADC 36 provide a DC feedback path to the microprocessor44, and the HPF 34, amplifier 40, and ADC 42 provide a feedback path tothe microprocessor 44 for the frequency band of the dither tones, inaccordance with which the microprocessor 44 controls the gain of theamplifier 52 to maintain a desired constant depth of modulation by theoptical modulator 24. For example, the modulation depth may be about 4%.The use of a constant modulation depth for channel identificationfacilitates determining optical power levels in the WDM network in knownmanner.

Although FIG. 4 a represents an arrangement for only one opticalchannel, it can be appreciated that the same arrangement can be providedfor each optical channel, and that parts of these respectivearrangements may be common for multiple optical channels. For example,it can be appreciated that the ADCs 36 and 42 and the microprocessor 44can be multiplexed for operation for a plurality of optical channels,the oscillator 46 can be used in common for all of the optical channels,and only as many DDSs 48 are required as the number of dither tones usedfor identifying the respective optical channels.

In this respect, it is observed that a particularly convenientarrangement can be provided by providing all of the components of FIG. 4a, except the modulated laser source 22, for each of a plurality ofoptical channels which are initially multiplexed by an opticalmultiplexer (similar to the multiplexer 10 of FIG. 1) at the inputs ofthis multiplexer.

This facilitates implementation of the arrangement of FIG. 4 a with themultiplexed operation as described above for the plurality of opticalchannels, while enabling the optical channels to be supplied fromarbitrary modulated laser sources such as the source 22. More generally,it can be appreciated that the dither tones can be applied to anyoptical channel anywhere between its source and its multiplexing withone or more other optical channels.

FIG. 5 illustrates a corresponding dither tone detection arrangement,which may be used at any desired point in the optical WDM network foridentifying an optical channel on an optical fiber or path 60 bydetecting the dither tones.

Referring to FIG. 5, an optical tap 62 supplies a small portion, e.g.5%, of an optical signal on the path 60 to an optical detector 64, andsupplies most of the optical signal power to an ongoing optical path 66.An electrical output of the optical detector 64 is amplified by acontrolled gain amplifier 68, an output of which is supplied via a bandpass filter (BPF) 70 and an amplifier 72 to an ADC 74. The BPF 70 has apass band including the dither tone frequency range. The ADC 74 producesa digital signal which represents detected dither tones and is suppliedvia a FIFO (first-in, first-out store) 76 to a digital signal processoror microprocessor 78. The microprocessor 78, which has an associatedmemory 80, controls the gain of the amplifier 68 in accordance with thepower of the optical signal on the path 60.

The microprocessor 78 operates in a known manner to perform FFTprocessing of the digital signals provided by the ADC 74, using thememory 80 for this FFT processing, in respective FFT periods T to detectany dither tone modulation of the optical signal on the optical path 60,with the FIFO 76 ensuring that data is not lost during FFT processing bythe microprocessor 78. As discussed above, this determines therespective dither tones, and hence the optical channel identification,in one FFT operation in many instances of typical optical signal powerlevels. For ensuring detection of dither tones for relatively low poweroptical channels even in the presence of one or more relatively highpower optical channels on the path 60, without increasing the FFT periodT, coherent averaging of the FFT results is carried out over aplurality, possibly a large number, of successive FFT periods T asdescribed above.

As indicated above, there is no requirement for synchronization between,for example, the operation of the selector 50 in the dither tonegeneration arrangement of FIG. 4 and the FFT periods of themicroprocessor 78 in the detection arrangement of FIG. 5. The continuousgeneration of each dither tone ensures that, regardless of theparticular timing of the selection of this dither tone by a selectorsuch as the selector 50, and regardless of the particular relativetiming of the FFT periods T used in a detection arrangement, thecoherent averaging over a plurality of such FFT periods will graduallyresult in accumulated results properly representing any dither toneswhich are present. As indicated above, such coherent averaging comprisesan accumulation of the FFT frequency bin results, or amplitudes inaccordance with their respective phase differences for successive FFTperiods, these phase differences being determined by the microprocessor78 of the detection arrangement from the dither tone frequencies and theknown period “t”.

As indicated above, it is conceivable to replace the alternating dithertones, as described above for identification of each optical channel, bysome other combination, such as a summation, of these dither tones, forexample by replacing the selector 50 of FIG. 4 a by a signal combiner orsummer. However, as also indicated above, this is not preferred becauseit results in relatively increased modulation depth of each opticalchannel, with corresponding eye closure for detection of the high speeddata signal carried by the optical channel.

In another embodiment of the invention, an additional feature ofsynchronizing switching of dither tones respectively generated andprocessed by encoding and decoding arrangements at all nodes in anetwork is provided, i.e. all encoding/decoding arrangements are usingthe same clock (e.g. global clock), and all of them provideswitching/alternating of dither tones at the same time and during sametime interval. It means that the steps of modulating and detecting therespective dither tones are synchronized so that time intervals ofmodulating and detecting the respective dither tone have same durationand start at the instant of switching dither tone frequencies. Thissituation is illustrated for two alternating tones f1 and f2 in FIG. 6a, and a global clock 58 is shown in dashed line in FIGS. 4 and 5. Tohave global timing in the network, it is possible to run, e.g., aNetwork Time Protocol (NTP) on a controlled network. Then a networkmanager block 59 running the network management software (NMS) sends theglobal/network timing information to the global clock 58 at each node.Respective synchronization signals are then sent to the microprocessors44 and 78 at the encoding and decoding arrangements to synchronize bothencoding and decoding processes in the network.

Such an encoding/decoding scheme using global synchronization has thefollowing advantages. It allows to detect dither tones easier and withhigher signal to noise ratio (about 3dB higher) as only one of thealternating dither tones appears in an FFT spectrum at a time (FIG. 6 b)instead of two tones in unsynchronized schemes (FIG. 6 c). Because thereis no frequency change in an FFT spectrum at a time,leakage/interference between the two (or more) dither tones ismitigated. It means that spectral range available for the assignment ofdither tones can accommodate more tones that will not interfere witheach other. As a result, more dither tone frequencies are available forassignment to optical signals, and the whole spectral range is used moreefficiently.

In yet another modification to the embodiments of the invention, theencoding of dither tones onto the optical channel can be made by adither tone generator and modulating arrangement which uses one DDSonly. Such an arrangement is illustrated by FIG. 7. It is similar to thearrangement of FIG. 4 a except for the selector 50 and one of the twoDDSs 48 being removed from the arrangement (retaining only one DDS), anda switching control 49 from the μP 44 going directly to the retained DDS48 (instead of going to the selector 50 in FIG. 4 a). The rest of thedither tone generator and modulating arrangement is identical to thatshown in FIG. 4 a, where the same elements are designated by the samereference numerals on both FIGS. 4 a and 7.

In the modulating arrangement of FIG. 7 the DDS 48 is directlycontrolled by the μP 44, i.e., it is controlled when to switch betweenselected dither tones. By having just one DDS 48, the phase continuitybetween the two dither tones of different frequencies is providedopposed to the arrangement of two DDSs which had a shift (jump) inphases between their respective tones. It is known that sudden phaseshift may result in unwanted high frequency noise (leakage) at DSP andbroadening of the dither tones themselves. Thus, by using just one DDS48 and providing phase continuity of dither tones, this problem isreduced or eliminated.

In one more modification to the embodiments of the invention, theencoding of dither tones onto the optical channel is performed by usinga dither tone generator and modulating arrangement having two DDSs,where a technique of “hardware windowing” has been implemented. Meansfor performing hardware windowing is illustrated in FIG. 8. It includesthe microprocessor μP 44, which provides control of DDS1 and DDS2(designated as 48), the DDS1 and DDS2 being further connected to amultiplier 51 via corresponding LPFs 34. DDS1 is to generate twoalternate dither tones having frequencies ƒ1 and ƒ2 respectively asillustrated in FIG. 9A. For example, ƒ1 and ƒ2 may be chosen as ƒ1=50000 Hz and ƒ2=50 010 Hz. DDS2 is to generate a very slow frequency toneƒs (e.g., 0.25 Hz), which is mixed with the ƒ1 and ƒ2 tones from DDS1 toresult in the dither tone profile shown in FIG. 9B. The μP 44 controlsthe switching between frequencies ƒ1 and ƒ2 in such a manner that thefrequency switching occurs right at the minimum point of tone ƒs, i.e.,when its amplitude is minimal and close to zero (see FIG. 9B). As aresult, the requirement for the phase continuity between ƒ1 and ƒ2 tonesmay be substantially reduced or eliminated. Because the frequencyswitching takes place when the amplitude of the dither signal is closeto zero, the phase shift (if there is any) does not contribute to theleakage at DSP between different FFT bins, and thus, performance of themodulating arrangement is substantially improved.

Although embodiments of the invention have been described with the useof Fast Fourier Transform (FFT), it is contemplated that other knowntypes of discrete transforms can also be used in the step of detectingand processing dither tones. Examples of such transforms are DiscreteCosine Transform (CDCT), Discrete Sinus Transform (DST), InverseDiscrete Cosine Transform (IDCT), Inverse Discrete Sinus Transform(IDST), Fast Walsh Transform (FWT), Fast Hadamard Transform (FHT).

Although embodiments of the invention have been described with the useof DDSs to generate dither tones, it can be appreciated that thewaveform synthesis can be accomplished through other ways. For example,in other implementations waveform synthesis is accomplished throughpre-computation of complex waveform tables that are sent to a Digital toAnalog Converter (DAC) for optical modulation, as opposed to the use ofDDS chips with an analog switch. An example implementation is depictedin FIG. 4 b. However, before describing how waveform synthesis can beaccomplished using a DAC further details of waveform synthesis usingDDSs will now be discussed in further details with reference to FIG. 4a.

To generate a dither signal having cyclically repeating tones offrequencies of ƒ1=10 kHz and ƒ2=10.050 kHz and having a periodicity of 2seconds for example, one of the DDSs 48 is programmed to output a signal90 at ƒ1=10 kHz and the other DDS 48 is programmed to output anothersignal 92 at ƒ2=10.050 kHz. In particular, each DDS 48 has registers(not shown), which store respective ones of the frequencies ƒ1 and ƒ2.The selector 50 switches between the signals 90, 92 every 2 seconds toproduce a combined signal 94 with alternating frequencies ƒ1 and ƒ2.

In FIG. 4 b, shown is another form of dither tone generator andmodulating arrangement in accordance with another embodiment of theinvention. The dither tone generator and modulating arrangement issimilar to that of FIG. 4 a except that the DDSs 48 and the selector 50are replaced with a DAC 49. For example, to generate a dither signalhaving cyclically repeating dither tones of frequencies of f₁=10 kHz andf₂=10.050 kHz with a periodicity of 2 seconds, the μP 44 computes a DACoutput value based on the following equation: DAC output value=S(t) sin(2*pi*10 kHz*t)+(1−S(t))sin(2*pi*10.050 kHz*t), where S(t) is atime-dependent function with a value of either 0 or 1. In thisparticular example the time-dependent function alternates between 0 and1 with a periodicity of 2 seconds. Computed DAC output values are thensent as digital data to the DAC 49 to generate a signal 96 havingcyclically repeating dither tones of frequencies of ƒ1=10 kHz andƒ2=10.050 kHz with a periodicity of 2 seconds.

Examples have been described in which a cyclically repeated sequence oftwo continuous dither tones is used to obtain a modulating signal foroptical modulation. For example, with reference to FIG. 6 a a cyclicallyrepeated sequence of two dither tones of frequencies ƒ1 and ƒ2 is used.This sequence can be described by the sequence of frequencies ƒ1, ƒ2, .. . , ƒ1, ƒ2, . . . In other implementations, other cyclic repeatedsequences are used. For example, as discussed above in someimplementations three dither tones with frequencies ƒ1, ƒ2, and ƒ3 forma cyclic repeated sequence which is used for modulating an opticalchannel. Such an sequence can be described by the sequence offrequencies ƒ1, ƒ2, ƒ3, . . . , ƒ1, ƒ2, ƒ3, . . . , for example. Moregenerally, two or more dither tones form a cyclic repeated sequence.Furthermore, in some implementations a particular tone is used more thanonce within the repeated sequence. For example, in one implementationthree dither tones with frequencies ƒ1, ƒ2, and ƒ3 form a cyclicrepeated sequence used for modulating an optical channel, and the tonewith frequency ƒ2 is used twice in the sequence. An example of suchimplementation can be described by the sequence of frequencies ƒ1, ƒ1,ƒ2, ƒ3, . . . , ƒ1, ƒ1, ƒ2, ƒ3, . . . , for example. More generally, insome implementations one or more dither tones are used more than oncewithin a sequence.

Example implementations have been described with reference to repeatedsequences wherein at any instant in time an optical channel is modulatedwith only one dither tone that is used for channel identification. Forexample, with reference to FIG. 6 a although a sequence of two dithertones with frequencies ƒ1 and ƒ2 is used for modulation, at any instantin time only one of the two dither signals is used to modulate anoptical channel. In some implementations two or more dither signals arecombined simultaneously and are used for channel identification. Anexample implementation will now be described with reference to FIG. 4 c.FIG. 4 c illustrates another form of dither tone generator andmodulating arrangement in accordance with another embodiment of theinvention. The arrangement of FIG. 4 c is similar to that of FIG. 4 aexcept that the arrangement in FIG. 4 c also has DDSs 49, a selector 51,and a summer 100.

The DDSs 49 produce dither tones with frequencies ƒ1 and ƒ2. Theselector 51 receives the dither tones from the DDSs 49 and alternatelyselects one of the dither tones in accordance with a pre-determinedperiodicity for transmission to the summer 100. The DDSs 48 producedither tones with frequencies ƒ3 and ƒ4. The selector 50 receives thedither tones from the DDSs 48 and alternately selects one of the dithertones in accordance with the pre-determined periodicity for transmissionto the summer 100. At one interval in time the summer 100 receives thedither tones of frequencies ƒ1 and ƒ3 and combines these dither tones toproduce a composite signal for modulating the optical channel. Atanother interval in time the summer 100 receives the dither tones offrequencies ƒ2 and ƒ4 and combines these dither tones to produce acomposite signal for modulating the optical channel. This process isrepeated according to the predetermined periodicity resulting a combinedsignal with frequencies alternating between (ƒ1, ƒ3) and (ƒ2, ƒ4), whichis used to produce a modulated optical channel in optical path 28. At aparticular interval in time the resulting modulated optical channel inthe optical path 28 is modulated with a set of two dither signals ofeither frequencies (ƒ1, ƒ3) or (ƒ2, ƒ4). The detection of such amodulated optical channel will now be described with reference to FIG.5.

In FIG. 5 the modulated optical channel is received on the path 60 andprocessed as discussed above. In particular, the microprocessor 78performs a sequence of frequency analysis operations, such as FFTprocessing of digital signals provided by the ADC 74 for example.However, for each operation within the sequence two dither signals maybe detected depending on the intensity of the optical channel, payloadnoise level, and interference tones. For example, for a particularwindow in time one or more of the dither signals of frequencies ƒ1, ƒ3is detected, whereas for another window in time one or more of thedither signals of frequencies ƒ2, ƒ4 is detected. In someimplementations the optical channel is identified when one or moredither tones from each set of dither tones is detected. In particular,in this example the channel is modulated with a cyclically repeatedsequence of combined dither tones with frequencies (ƒ1, ƒ3), (ƒ2, ƒ4), .. . , (ƒ1, ƒ3), (ƒ2, ƒ4), . . . However, the optical channel may be tooweak for detection of all of the tones. For example, if the dither toneof frequency ƒ1 is not detected during a collision of an interferencetone, the following cyclically repeated sequence will be detected (ƒ3),(ƒ2, ƒ4), . . . , (ƒ3), (ƒ2, ƒ4), . . . In the example implementation,the channel is identified when such a sequence is detected. Table Ilists the cyclically repeated sequences of dither tones, which are usedto identify a channel modulated with the cyclically repeated sequence(ƒ1, ƒ3), (ƒ2, ƒ4), . . . , (ƒ1, ƒ3), (ƒ2, ƒ4), . . . TABLE I List ofCyclically Repeated Sequences used to detect a channel. Sequence NumberCyclically Repeated Sequence Detected 1 (f1, f3), (f2, f4), . . . , (f1,f3), (f2, f4), . . . 2 (f1), (f2, f4), . . . , (f1), (f2, f4), . . . 3(f3), (f2, f4), . . . , (f3), (f2, f4), . . . 4 (f1, f3), (f4), . . . ,(f1, f3), (f4), . . . 5 (f1, f3), (f2), . . . , (f1, f3), (f2), . . . 6(f1), (f2), . . . , (f1), (f2), . . . 7 (f3), (f4), . . . , (f3), (f4),. . . 8 (f1), (f4), . . . , (f1), (f4), . . . 9 (f3), (f2), . . . ,(f3), (f2), . . .

In Table I, for Sequence 1 all of the dither tones are detected. ForSequences 2 to 5 only one of the dither tones from one of the sets ofdither tones is detected, and for Sequences 6 to 9 only one of the twodither tones detected in each set. In the example, the channel isidentified when any one of Sequences 1 to 9 is detected. This has theadvantage that an optical channel is still identified even if one ormore of the dither tones is not detected, thereby providing a robustdetection method. In other implementations the optical channel isidentified only when both dither tones from a particular set of dithertones are detected. In the above implementations, dither tones aredistinguished from interference tones. For example, the detection of theswitching of dither tones of frequencies ƒ1 and ƒ2, for example, atevery 2 seconds over a period of time provides very conclusiveinformation that the detected dither tones are indeed dither tonesrather than payload interference tones. This way, in additions tomitigating problems with false negative detection where a channel ispresent but is missed, problems with false positive detection where achannel is not present but is falsely detected are also mitigated.

In the above example, the cyclic repeated sequence of dither tones hastwo sets of dither tones. However, it is to be clearly understood thatother implementations are possible. More generally, each set in thesequence has one or more dither tones, and a channel is identified whenone or more dither tones from each set is detected. For this purpose, itis to be clearly understood that in some embodiments of the inventionthe arrangement of FIG. 4 c has one or more DDSs, none or moreselectors, and one or more summers. In yet other embodiments of theinvention, one or more DACs are used to produce combined dither signals.

In the above example, each set of dither signals within a cyclicrepeated sequence of dither tones has two dither tones. One of the setshas dither signals of frequencies ƒ1, ƒ3 and the other set has dithertones of frequencies ƒ2, ƒ4. In some implementations, the number ofdither tones varies from set to set within the sequence. For example, insome implementations a cyclic repeated sequence of dither tones has aset of 3 dither tones of frequencies ƒ1, ƒ2, ƒ3 combined simultaneouslyto modulate an optical channel within an interval time and another setof 2 dither tones of frequencies ƒ4, ƒ5 combined simultaneously tomodulate the optical channel within another interval time.

Embodiments of the invention have been described in which a combinationof two or more dither tones are used for marking and detecting anoptical channel. However, it is to be clearly understood thatembodiments of the invention are not limited to using dither tones forpurposes of optical channel detection. In some embodiments of theinvention dither tones are also used in signal detection for detectingother network parameters such as links or fiber sections carrying theoptical channel, an optical band including the channel, a source node inthe network where the channel is marked, a destination node in thenetwork where the channel is detected, an optical fiber carrying thechannel, and bundle of fibers over which the optical channel is carried,for example.

For example, in an example implementation a first cyclically repeatedsequence of dither tones with frequencies ƒ1, ƒ2 is used to identify anoptical channel and a second cyclically repeated sequence of dithertones with frequencies ƒ3, ƒ4 is used to identify an optical fiber overwhich the optical channel is being transmitted. In some implementationsthe sequences are modulated simultaneously. This is illustrated in TableII where the sequences of dither tones are shown modulatedsimultaneously within each time interval t_(i), where i=1, 2, 3, . . .Network Time Interval Parameter t₁ t₂ t₃ t₄ t₅ Channel f1 f2 f1 f2 f1 IDFiber ID f3 f4 f3 f4 f3Table II: List of cyclically repeated sequences of dither tones used todetect a channel and a fiber, the cyclically repeated sequences beingmodulated simultaneously.

In Table II, the cyclically repeated sequences of dither tones aremodulated simultaneously. In particular, within a time interval t_(i)either the dither tones of frequencies ƒ1 and ƒ3 or the dither tones offrequencies ƒ2 and ƒ4 are modulated simultaneously.

In other implementations the dither tones from the first and secondcyclically repeated sequences of dither tones are modulatedsequentially. For example, in Table III the dither tones of frequenciesƒ1, ƒ2, which are used for channel detection, are modulated at timeintervals t_(1,) t_(2,) t_(5,) t_(6,) . . . . On the other hand, thedither tones of frequencies ƒ3, ƒ4, which are used for fiberidentification, are modulated at time intervals t_(3,) t_(4,) t_(7,)t_(8,) . . . . TABLE III List of cyclically repeated sequences of dithertones used to detect a channel and a fiber, the cyclically repeatedsequences being modulated sequentially. Time Intervals and NetworkParameters Channel Channel Fiber Fiber Channel Channel Fiber Fiber ID IDID ID ID ID ID ID t₁ t₂ t₃ t₄ t₅ t₆ t₇ t₈ f1 f2 f3 f4 f1 f2 f3 f4

The above examples make use of two cyclically repeated sequences ofdither tones for channel and fiber identification. However, in otherimplementations more than two cyclically repeated sequences of dithertones are modulated on an optical channel, and each sequence is used insignal detection to detect any suitable network parameter. Furthermore,the cyclically repeated sequences of dither tones are modulatedsimultaneously, sequentially, or using any suitable combinationsimultaneous and sequential modulation. For example, in yet anotherimplementation at each time interval one or more dither tones ismodulated and used for identifying a respective network parameter. Suchan implementation will now be described with reference to Table IV.TABLE IV List of cyclically repeated sequences of dither tones used todetect a channel and a fiber. Time Intervals and Network ParametersChannel Fiber Channel Fiber Channel Fiber Channel Fiber ID ID ID ID IDID ID ID t₁ t₂ t₃ t₄ t₅ t₆ t₇ t₈ f1, f2 f3, f4 f1, f2 f3, f4 f1, f2 f3,f4 f1, f2 f3, f4

In Table IV the dither tones of frequencies ƒ1, ƒ2, which are used forchannel detection, are modulated at time intervals t₁, t₃, t₅, t₇, . . .. On the other hand, the dither tones of frequencies ƒ3, ƒ4, which areused for fiber identification, are modulated at time intervals t₂, t₄,t₆, t₈, . . . . In this example implementation, at each time intervaltwo dither signals are used to identify a respective network parameter.More generally, at each time interval for each of one or more networkparameters one or more dither signals are modulated.

Although the description above relates to an optical WDM network inwhich optical channels are identified by having their intensitymodulated with a combination of a plurality of dither tones, it can beappreciated that similar principles can be applied to identifying anddetecting multiple channels in networks and systems using other types ofcommunications media and modulation methods. Other modulating techniquesinclude amplitude modulation, frequency modulation, phase modulation,and polarization modulation for example.

Thus although particular embodiments of the invention are describedabove, it can be appreciated that these and numerous othermodifications, variations, and adaptations may be made without departingfrom the scope of the invention as defined in the claims.

1. A method of identifying a signal in an optical communications networkcomprising the steps of: modulating an optical signal with a cyclicallyrepeated sequence of low frequency dither tones having a predeterminedperiodicity; performing a sequence of frequency analysis operations onthe optical signal to produce respective frequency analysis results; anddetecting the low frequency dither tones using the respective frequencyanalysis results produced by the frequency analysis operations.
 2. Themethod according to claim 1 wherein the step of marking comprisesallocating one or more dither tones for identifying a network parameterof the optical communications network.
 3. The method according to claim1 wherein the cyclically repeated sequence of low frequency dither tonescomprises at least one set of low frequency dither tones modulatedsimultaneously, the method comprising: identifying a network parameterby detecting at least one of the low frequency dither tones modulatedsimultaneously from each set.
 4. A method according to claim 1 whereinthe optical signal is modulated with at least one other cyclicallyrepeated sequence of low frequency dither tones having a respectivepredetermined periodicity, the method further comprising: for each othercyclically repeated sequence of low frequency dither tones: detectingthe low frequency dither tones using the respective frequency analysisresults produced by the frequency analysis operations, and; identifyinga respective network parameter from the detected low frequency dithertones.
 5. A method according to claim 4 wherein each cyclically repeatedsequence of low frequency dither tones uniquely identifies saidrespective network parameter.
 6. A method according to claim 4 whereineach network parameter comprises one of a channel ID (IDentifier), alink ID, an optical fiber ID, a fiber section ID, an optical band ID, asource node ID, a destination node ID, and a bundle ID.
 7. A methodaccording to claim 4, wherein the cyclically repeated sequences of lowfrequency dither tones are modulated in one of the following ways: (a)simultaneously; (b) consecutively; and (c) using a combination of (a)and (b).
 8. A method according to claim 1 wherein each low frequencydither tone in the cyclically repeated sequence of low frequency dithertones is repeated with substantially the same phase at saidpredetermined periodicity, the method comprising: performing coherentaveraging of the frequency analysis results over a plurality offrequency analysis operations to detect the low frequency dither tones.9. A method of modulating an optical signal in an optical communicationsnetwork comprising: modulating the optical signal with a cyclicallyrepeated sequence of low frequency dither tones having a predeterminedperiodicity.
 10. A method according to claim 9 wherein the cyclicallyrepeated sequence of low frequency dither tones comprises at least oneset of low frequency dither tones modulated simultaneously.
 11. A methodaccording to claim 9 comprising: modulating the optical signal with atleast one other cyclically repeated sequence of low frequency dithertones and with a predetermined periodicity.
 12. A method according toclaim 9 wherein each cyclically repeated sequence of low frequencydither tones uniquely identifies a respective network parameter.
 13. Amethod according to claim 12 wherein each network parameter comprisesone of a channel ID (IDentifier), a link ID, an optical fiber ID, afiber section ID, an optical band ID, a source node ID, a destinationnode ID, and a bundle ID.
 14. A method according to claim 11, whereinthe cyclically repeated sequences of low frequency dither tones aremodulated in one of the following ways: (a) simultaneously; (b)consecutively; and (c) using a combination of (a) and (b).
 15. A methodaccording to claim 9 wherein at least one of the low frequency dithertones is modulated at least twice within the sequence.
 16. A methodaccording to claim 9 wherein the cyclically repeated sequence of lowfrequency dither tones comprises a plurality of sets of at least one lowfrequency dither tone modulated simultaneously, at least two of theplurality of sets having different numbers of low frequency dithertones.
 17. A detection arrangement for an optical communications networkcomprising: a detector for detecting an optical signal modulated with acyclically repeated sequence of low frequency dither tones having apredetermined periodicity; and a processor for performing a sequence offrequency analysis operations on the optical signal to producerespective frequency analysis results and for detecting the lowfrequency dither tones using the respective frequency analysis resultsproduced by the frequency analysis operation.
 18. An arrangementaccording to claim 17 wherein the processor comprises means foridentifying a network parameter from the detection of the low frequencydither tones.
 19. An arrangement according to claim 17 wherein thecyclically repeated sequence of low frequency dither tones comprises atleast one set of low frequency dither tones modulated simultaneously,the processor comprising means for identifying a network parameter bydetecting at least one of the low frequency dither tones modulatedsimultaneously from each set.
 20. An arrangement according to claim 17wherein the optical signal is modulated with at least one othercyclically repeated sequence of low frequency dither tones having arespective predetermined periodicity, for each other cyclically repeatedsequence of low frequency dither tones the processor comprising: meansfor detecting the low frequency dither tones using the respectivefrequency analysis results produced by the frequency analysisoperations, and; means for identifying a respective network parameterfrom the detected low frequency dither tones.
 21. An arrangementaccording to claim 20 wherein each cyclically repeated sequence of lowfrequency dither tones uniquely identifies said respective networkparameter.
 22. An arrangement according to claim 18 wherein each networkparameter comprises one of a channel ID (IDentifier), a link ID, anoptical fiber ID, a fiber section ID, an optical band ID, a source nodeID, a destination node ID, and a bundle ID.
 23. An arrangement accordingto claim 20, wherein the cyclically repeated sequences of low frequencydither tones are modulated in one of the following ways: (a)simultaneously; (b) consecutively; and (c) using a combination of (a)and (b).
 24. An arrangement according to claim 17 wherein each lowfrequency dither tone in the cyclically repeated sequence of lowfrequency dither tones is repeated with substantially the same phase atsaid predetermined periodicity, the processor comprising means forperforming coherent averaging of the frequency analysis results over aplurality of frequency analysis operations to detect the low frequencydither tones.
 25. A modulating arrangement for modulating an opticalsignal, the modulating arrangement comprising: means for producing acyclically repeated sequence of low frequency dither tones having apredetermined periodicity; and a modulator for modulating the opticalsignal with the cyclically repeated sequence of low frequency dithertones.
 26. An arrangement according to claim 25 wherein the cyclicallyrepeated sequence of low frequency dither tones comprises at least oneset of low frequency dither tones modulated simultaneously.
 27. Anarrangement according to claim 24 comprising means for producing atleast one other cyclically repeated sequence of low frequency dithertones having a predetermined periodicity, the modulator comprising meansfor modulating the optical signal with the at least one other cyclicallyrepeated sequence of low frequency dither tones.
 28. An arrangementaccording to claim 24 wherein the means for producing a cyclicallyrepeated sequence of low frequency dither tones comprises at least oneprogrammable frequency source for generating the low frequency dithertones.
 29. An arrangement according to claim 24 wherein the means forproducing a cyclically repeated sequence of low frequency dither tonescomprises at least two programmable frequency sources for generating thelow frequency dither tones and at least one selector for alternatelyselecting the low frequency dither tones.
 30. An arrangement accordingto claim 24 wherein the means for producing a cyclically repeatedsequence of low frequency dither tones comprises a DAC (Digital toAnalog Converter) for generating the low frequency dither tones.
 31. Anoptical system comprising: a modulating arrangement for generating acyclically repeated sequence of low frequency dither tones having apredetermined periodicity and for modulating an optical signal with thecyclically repeated sequence of low frequency dither tones, and; adetection arrangement for: detecting the optical signal; performing asequence of frequency analysis operations on the optical signal toproduce respective frequency analysis results, and; detecting the lowfrequency dither tones using the respective frequency analysis resultsproduced by the frequency analysis operation.
 32. A system according toclaim 31 comprising a global clock for synchronizing time intervals formodulating the optical signal with the low frequency dithers tones atthe modulating arrangement and time intervals for detection of the lowfrequency dither signals at the detection arrangement.